1. Field of the Invention
The present invention relates to a linear voltage regulator for supplying a load while maintaining the voltage across the load at a predetermined value. The present invention more specifically applies to a voltage regulator for supplying, by means of a battery, a mobile device. In particular, the present invention applies to a voltage regulator for supplying a mobile telephone from a rechargeable battery.
2. Discussion of the Related Art
FIG. 1 shows an example of conventional diagram of a positive voltage linear regulator circuit.
Such a regulator essentially includes an amplifier 1 for controlling a power element 2 meant to supply a load 3 under a predetermined voltage Vout. An input terminal E of the regulator is connected to a rechargeable battery 4 for supplying the power required for the supplying of load 3 connected to an output terminal S of the regulator. Power element 2 is generally formed of a MOS transistor, for example, a P-channel transistor, for minimizing, with respect to the use of a bipolar transistor, the so-called waste voltage, that is, the voltage drop between terminals E and S of the regulator. The source of transistor 2 is connected to terminal E while its drain forms terminal S. A decoupling capacitor C is generally connected between terminal S and the ground. Amplifier 1 includes a first inverting input connected to a terminal R of the regulator to which is applied a reference voltage Vref. A second non-inverting input of amplifier 1 is connected to terminal S. An output of amplifier 1 is connected to the gate of transistor 2 to modify, according to the error voltage between the inverting and non-inverting inputs, the gate-source voltage of transistor 2 and thus maintain voltage Vout at reference value Vref. It is possible to have a resistive dividing bridge (not shown) interposed between terminal S and the inverting input of amplifier 1 to introduce a constant coefficient of proportionality between voltages Vref and Vout.
If the regulator is meant for supplying a mobile device from a rechargeable battery, it is desirable for the regulator to have the lowest possible power consumption to improve the charge hold duration of the battery. Further, the regulator must then have a waste voltage which is the lowest possible which depends, in particular, on the drain source resistance in the on-state of the transistor.
A disadvantage of a regulator such as shown in FIG. 1 is that the power consumption by the regulator is substantially constant despite changes in the current consumed by load 3. For example, in an application to mobile phones, where the current surge for the regulator is about one hundred milliamperes during a communication and from about 100 .mu.A to a few mA while waiting for a communication, the power consumption of a conventional regulator is about 200 .mu.A.
To reduce the power consumption of the regulator when the load only requires a low current, a self-biased amplifier 1, the biasing current of which adapts according to the load, is generally used. If such a solution enables making the consumption of the regulator variable according to the load, this consumption remains high (for example, about 100 .mu.A) for a low output current. Indeed, if the power consumption of amplifier 1 decreases, the resistance of the output stage (generally formed of a MOS transistor) of amplifier 1 increases, which causes a lowering of the cut-off frequency linked to the gate capacitance of MOS power transistor 2. Indeed, transistor 2 generally is a transistor of significant size, that is, having a high ratio W/L (gate width over length) and thus has a high gate capacitance. Thus, if it is attempted to decrease the power consumption of the amplifier caused by its self-biasing too much, the circuit becomes unstable since the pole connected to the gate capacitor of transistor 2 becomes neighbor to that connected to decoupling capacitor C. To make the loop stable, the different poles always have to be distant from one another. A solution consists of multiplying the number of stages, but the power consumption is then increased.
The problem of stability is particularly critical in a linear regulator because, differently from other power converters of switched-mode supply type where the reverse feedback controlling the output voltage is discontinuous, the reverse feedback loop in a linear regulator is permanent.
A compromise thus has to be made between the decrease of the consumption of amplifier 1 for a low output current and the stability of this regulator. Referring to the above example of a mobile phone, this results, in practice, in a minimum current consumption of about 100 .mu.A for the regulator.
It would be desirable to further decrease this minimum consumption to improve the charge hold duration of battery 4.